Abstract
In the present paper, we have examined the photophysics and the photochemical oxidation of the indole alkaloid yohimbine in moderately concentrated sulfuric acid methanol–water media. The primary photophysical processes of yohimbine resemble those of indole. The differences can mainly be attributed to the presence of its exocyclic piperidinic ring. In the presence of light, the acid solutions of yohimbine are sensitive to atmospheric oxygen and they oxidize to give 3,4-dehydroyohimbine. The disappearance of yohimbine and the formation of dehydroyohimbine follow first order kinetics whose rate constants increase linearly with the concentration of acid and with the intensity of the exciting radiation. A two-step mechanism is proposed for this photochemical reaction. In the first step, excited yohimbine reacts with ground state oxygen to give hydroperoxoindolenine. This intermediate slowly rearranges in a second acid catalyzed step to yield 3,4-dehydroyohimbine.
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